In order to create a sensitive electrodynamic measuring device, it is customary to provide a high input impedance and thereby reduce the power of the input signal required to operate the device. However, electronic circuits with a very high input impedance tend to be unstable, and so practical devices are usually a compromise between achieving the necessary degree of sensitivity, providing the desired input impedance and ensuring an acceptable degree of stability.
In International Patent Application No. WO 03/048789, an electrodynamic sensor is disclosed in which a number of different circuit techniques are combined to achieve several orders of magnitude improvement in sensitivity, by comparison with previously known electrodynamic sensors, whilst still maintaining sufficient stability to permit a relatively unskilled operator to make measurements in everyday conditions. According to this earlier application, an electrodynamic sensor is provided comprising a high input impedance electrometer, which is adapted to measure small electrical potentials originating from an object under test and which employs at least one input probe having no direct electrical contact with the object. The circuit arrangement of the electrometer of this invention comprises an amplifier, which includes a combination of ancillary circuits providing feedback from the output of the amplifier and arranged cumulatively to increase the sensitivity of the electrometer to the small electrical potentials whilst not perturbing the electrical field associated therewith, the ancillary circuits serving to provide at least two of: guarding, bootstrapping, neutralisation, supply rail drift correction, supply modulation and offset correction for said sensor.
Whilst these features assist in providing a sensor with high input impedance and a relatively stable operation, nevertheless, in situations where there may be weak capacitive coupling to, or a signal of small amplitude generated by, a source or sample under test, noise problems may still remain and may inhibit or prevent accurate signal measurement. This is particularly the case in certain medical and microscopic applications in which there is only a weak capacitive coupling and yet highly accurate signal measurement is essential, for example in a remote off-body mode of sensing in which the or each probe has no physical contact with the human body and typically the weak capacitive coupling would be <1 pF.
More particularly, in applications where there is a weak coupling between a sample under test and the sensor electrode, the capacitive coupling to the sample may be comparable with or much smaller than the input capacitance of the sensor. In this case, the measurement signal received by the sensor is attenuated by the capacitive potential divider formed by the coupling capacitance and the input capacitance and may be difficult to capture.
Furthermore, the use of the output signal from the amplifier as the feedback signal has the disadvantage that such a signal is a broadband signal, which may have a poor signal to noise ratio. Hence, the noise is then fed back to the amplifier input with the feedback signal, causing further degradation of the signal to noise ratio.
There is thus a significant need for an electric potential sensor in which the possibility for accurate signal measurement is enhanced in cases of weak capacitive coupling to a sample under test.
Such a need is especially pronounced in cases where accuracy of signal measurement is critical.
There is also a significant need for an electric potential sensor in which the signal to noise ratio is substantially improved.